The present invention relates to an operating method of a boiling water reactor and, more particularly, to an operating method of a boiling water reactor in the event that the flow rate of coolant through the core (referred to as "core flow rate", hereinunder) is decreased due to, for example, a trip of the circulating pump.
The specifications of U.S. Pat. Nos. 4,108,720 and 3,565,760 disclose operating methods of nuclear reactors. During operation of a nuclear reactor in accordance with these methods, when the core flow rate is decreased accidentally due to a trip of the circulating pump, a high core output is maintained despite of the reduced core flow rate so that a nuclear-hydrothermally unstable condition is caused in the nuclear reactor. Hitherto, in order to avoid such an unstable condition, it has been a common measure to stop the operation by a scram or to continue the operation with reduced output by inserting preselected control rods, when the circulating pump is tripped.